The present disclosure relates to a protective circuit, which is configured to protect a circuit from electrostatic discharge resulting from static electricity from the outside. The present disclosure also relates to an electronic device including the protective circuit, and a method of driving the protective circuit.
Wiring embedded in semiconductor integrated circuits such as LSIs (Large Scale Integrations) is becoming finer. Drive voltages of the semiconductor integrated circuits are becoming lower. In general, as a result, it is becoming more important to protect embedded circuits from surge currents. The surge current occurs in a power-source line in the semiconductor integrated circuits. Note that, in the present specification, an embedded circuit to be protected will be referred to as “protected circuit”.
ESD surge is known as a typical surge current, which occurs in a power-source line. The following phenomenon is referred to as ESD surge. That is, electro-static discharge (ESD) to a terminal outside of a power-source line increases a voltage of the power-source line suddenly.
In the past, protective circuits, i.e., so-called global clamp and local clamp, are known. Such protective circuits are configured to protect protected circuits from ESD surge. The global clamp is also referred to as primary clamp. The global clamp provides the following technology. That is, a protective circuit is provided between a power-source line and a ground potential section. As a result, the entire circuit of a predetermined area is protected. The local clamp is also referred to as secondary clamp. The local clamp provides the following technology. That is, the local clamp protects the predetermined protected circuit. The local clamp plays a supplementary role to the global clamp.
For example, in a circuit including a global clamp, when ESD surge flows in a power-source line, devices of a global clamp are driven. Then, a surge current is caused to flow to the ground potential side. Here, a path in which a surge current flows includes a resistance. Because of this, if the amount of current increases, a potential difference occurs between the both ends of the path. If the potential difference increases, the gate potential of a transistor of a protected circuit increases. As a result, a transistor is destroyed.
In order not to destroy devices as described above, a local clamp is provided. If a local clamp is provided, for example, in a case where the gate potential of a transistor of a protected circuit increases and reaches a predetermined voltage or more, devices of the local clamp are driven. As a result, the gate potential of the transistor may decrease.
Japanese Patent Application Laid-open No. 2008-98587 discloses that a MOS transistor (field-effect transistor) plays a role of a local clamp.